1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device in which element isolation is performed by forming an insulation film such as an oxide film between element regions.
2. Description of the Related Art Oxidation of Silicon) technique has
A LOCOS (Local been well-known as a method for isolating elements in a high-density semiconductor integrated circuit. The LOCOS technique is as follows: The surface of a semiconductor substrate is oxidized to form a surface-protecting insulation film. An oxidation-resistance film, e.g., a silicon nitride film is formed on the insulation film and it is then subjected to patterning. The semiconductor substrate is selectively oxidized using the silicon nitriding film as a mask thereby to form a thick element isolating insulation film on the substrate.
In the LOCOS technique, when the selective oxidization is carried out, an inserting portion of the element isolating insulation film, called a bird's beak, is formed below the silicon nitride film which is used as a mask. The bird's beak causes an error in dimension between the silicon nitride film and element isolating insulation film. The error in dimension particularly depends on the thickness of the surface-protecting insulation film. The thicker the surface-protecting insulation film, the larger the bird's beak. For example, in a semiconductor device using a silicon substrate as the semiconductor substrate and a silicon oxide film as the surface-protecting insulation film, if the thickness of a silicon nitride film is 250 nm, the thickness of the silicon oxide film is 150 nm, the thickness of an element isolating insulation film obtained before removing the silicon nitride film and silicon oxide film is 800 nm and the thickness of the insulation film obtained after removing them is 500 to 600 nm, the error in dimension is about 1.2 to 1.6 .mu.m. For this reason, in order to obtain electrically satisfactory element isolation characteristics using the LOCOS technique, the width of a practical element isolation region is at least 2.0 .mu.m and it cannot be made smaller than 2.0 .mu.m.